In the field of communication optical semiconductor devices, rather than the development of an optical device having a single function such as a Fabry-Perot laser device, a distributed feedback (DFB) laser device, a semiconductor optical amplifier and an optical modulator, attention has been paid to the development of a multifunction integrated optical device formed on a single substrate. A semiconductor buried waveguide is suitable for a waveguide of a current injection type optical semiconductor device such as a semiconductor laser device and a semiconductor optical amplifier, the semiconductor buried waveguide having a semiconductor filling material that fills either side of a mesa (convex) waveguide. As this semiconductor buried waveguide is adopted, stress contained in each region of the waveguide becomes small, so that it becomes possible to ensure high long-term reliability.
A so-called high mesa waveguide is effective for an optical modulator operating at high speed of several GHz to several tens GHz, the high mesa waveguide having a mesa waveguide, either side of which is filled with air or organic material of a low refractive index. Air and a low refractive index organic material have a dielectric constant lower than that of a semiconductor filling material, so that parasitic capacitance can be reduced. The high mesa waveguide is also suitable for a multimode interference waveguide, a curved waveguide and the like. Since either side of a waveguide is filled with a low refractive index material, it becomes possible to obtain high optical confinement effects. It is therefore possible to realize miniaturization and low loss.
An optical device has been proposed, integrating a buried waveguide and a high mesa waveguide on one substrate (Patent Documents 1 to 4). As the buried waveguide and the high mesa waveguide are coupled, a guided mode mismatch occurs due to discontinuity of refractive indices. The guided mode mismatch may cause reflection and scattering of guided light. As reflected light returns to a semiconductor laser device, the operation characteristics of the semiconductor laser device may be deteriorated.
It is possible to relax the guided mode mismatch by introducing a taper structure to a connection region between a buried waveguide and a high mesa waveguide (Patent Document 5).